It's been a long time since I originally asked this question. While my contributing answer here doesn't really clear up why draught beer (maybe, maybe doesn't) taste better than bottled beer, two things are clear to me now after spending a lot of time reflecting on the difference:
As Tom pointed out in the very first comment, drinking beer (and coffee too, for that matter!) out of a small spout (bottle, travel mug, etc.) blocks your nose from inhaling the aromas of the beer. This limits the amount of flavour you can sense. Pouring the bottled beer into a glass dramatically improves its flavour because the open mouth of the glass allows you to taste and smell.
There's an inverse relationship between perceived "better" taste and the volume of bottles a brewery produces (and how far they intend to distribute).
Beer brewed at a local microbrew has a shelf-life of only a few weeks on the outside; preservatives are limited or omitted and their ingredients are typically fresh, with a comparably short self-life.
Larger breweries distribute further distances, demand a longer shelf-life, and usually have a need to maintain a consistent taste across their entire production line and the resulting shelf-life. These breweries use high-volume equipment (an increase in the amount the beer is processed) and high-volume ingredients (preservatives in ingredients make them last longer, but may not be fresh). It also becomes increasingly difficult to quality-control when mass-producing — these breweries are less likely to take risks, and they rely on machinery to achieve consistency, often sacrificing quality.
tl;dr: draught beer, like micro-brewed beer was never intended to last for months or years on the shelf, and thus preservatives don't need to be factored into the production, and the beer is liable to be fresher more flavourful.
One last bonus point
I've also noticed that at times, bottled microbrews, poured into a glass can actually taste better than their draught counterparts. Go figure ¯\_(ツ)_/¯